A direct current motor with brushes has been generally used for an electric power steering system. An electric power steering system with a brushless motor has also been developed, which solves a problem of aged deterioration associated with a direct current motor with brushes. A brushless motor uses a three phase winding as an outer stator and plural permanent magnets as an inner rotor. When the three phase winding is supplied with current depending upon the rotational phase of inner rotor, the inner rotor will rotate. In this way, since a brushless motor does not require a brush, it is free from degradation of steering feeling caused by the abrasion of brush. A brushless motor, whose inner rotor is made of magnets, has a small moment of inertia, thereby preventing degradation of steering feeling caused by a big moment of inertia.
An apparatus for controlling an electric power steering system, which employs a brushless motor, sends a signal for motor control to a motor drive circuit so that the signal drives Field Effect Transistors (FET's) with Pulse Width Modulation (PWM) or switches them off. The apparatus thus determines a signal for target current based upon a signal of steering torque sent by a sensor for detection of steering torque and also compensates this signal for target current by inertia and damper control. These processes are performed so that assist steering torque generated by the brushless motor, which corresponds to target current running through the brushless motor, can be determined. Also the apparatus determines a signal for motor control based upon both a deviation between a signal for target current and a signal of motor current (actual current) sent by a device for detection of motor current, and a phase signal of motor rotation (an actual rotational phase of inner rotor) sent by a device for detection of rotational phase of motor, thereby supplying a target current to the brushless motor. In a motor drive circuit, the FET's are PWM driven based upon the signal for motor control, thereby rotating the brushless motor in a positive or reverse direction.
In the motor drive circuit, the FET's generate heat as a result of some ten amperes of current running therethrough. Also driving of a brushless motor requires accurate control of current depending on the rotational phase of an inner rotor. Two of relatively inexpensive and simple microcomputers, which share control of a brushless motor, are used in an apparatus for controlling an electric power steering system. A current control unit having a microcomputer which determines target current is placed apart from a motor drive circuit. In this way the current control unit can determine accurate target current since analogue circuits and the like for shaping a signal for steering torque are free from the effect of heat. On the other hand, a drive control unit having a microcomputer which determines a signal for motor control is placed in the neighborhood of a brushless motor and device for detection of rotational phase of motor. The drive control unit can thus perform accurate current control for the brushless motor based upon a phase signal of motor rotation which has a small amount of noise and no phase delay as a result of a short transmission path of the signal. These current control and drive control units are electrically coupled by wires, thereby communicating each other. For example, the current control unit sends a signal for target current, and on the other hand the drive control unit sends a phase signal of motor rotation to be used for damper control.
In a general control apparatus, in which plural microcomputers perform shared computation, the microcomputers are electrically connected by wires and various types of synchronization are adopted for mutual communication of data. An apparatus for controlling an electric power steering system has two microcomputers which share the drive control of a brushless motor and perform high-speed communication by clock synchronization.
In this apparatus, a microcomputer for a current control unit serves as a master microcomputer and the other microcomputer for a drive control unit serves as a slave microcomputer. The microcomputer in the current control unit thereby generates clock signals for communication by clock synchronization and transmits the signals to the microcomputer in the drive control unit via wires for clock signals. In this way, the two microcomputers transmit or receive data mutually via the wires.
However, when one of the two microcomputers fails, the apparatus cannot continue a sequence of control which is required by an electric power steering system. The apparatus is thus unable to perform drive control for a brushless motor, thereby failing to exert assist steering torque on a steering line. Also, when the current control unit cannot receive a phase signal of motor rotation due to a disconnection of wires for communication between the current and drive control units, the current control unit cannot perform damper control anymore, thereby damaging steering feeling of a driver.
Further, the microcomputer in current control unit requires data associated with a center value of steering torque, and on the other hand the microcomputer in drive control unit needs an offset value of motor encoder. In this way, each microcomputer reads out the data stored in an Electrically Erasable Programmable Read Only Memory (hereinafter referred to as EEPROM), which is prepared for each microcomputer. Then, the apparatus, which controls a brushless motor with two microcomputers, requires an EEPROM for each microcomputer, thereby resulting in a costly configuration with two EEPROM's.
Signals are transmitted or received by clock synchronization to perform high-speed communication between the two microcomputers. In this communication by clock synchronization, a master microcomputer (a microcomputer in current control unit) transmits data in reference to clock signals generated therein, and on the other hand a slave microcomputer (a microcomputer in drive control unit) receives the data in reference to the clock signals transmitted by the master microcomputer. The slave microcomputer will continue receiving unsynchronized data once a time lag occurs in the received data. This results in a situation that the slave microcomputer cannot receive normal data such as target current, and thereby the slave computer is unable to determine a correct signal for motor control. As a result, the control apparatus for an electric power steering system cannot exert assist steering torque on a steering line.
If the microcomputer in drive control unit is not ready to receive data, a time lag will appear in received data. This type of phenomenon occurs, for example, when components of the microcomputer in drive control unit are not electrically initiated or the initial check of a Central Processing Unit (CPU) has not been completed at starting of a vehicle (turning on of ignition switch). Also when a noise is on a wire running from the microcomputer in current control unit to the microcomputer in drive control unit while data is transmitted, a time lag appears in the data. The microcomputer in drive control unit thus receives unsynchronized signals.
Further, when the slave microcomputer is unable to receive correct clock signals due to a disconnection of wires for clock signals or failure of an interface circuit (port and the like), the master and slave microcomputers cannot communicate data by clock synchronization. In this way, the microcomputer in drive control unit cannot receive correct data (target current and the like) and thereby the apparatus for an electric power steering system cannot exert assist steering torque on a steering line.